Sulfur removal process

ABSTRACT

A product of reduced sulfur content is produced from an olefin-containing hydrocarbon feedstock which includes sulfur-containing impurities. The feedstock is contacted with an olefin-modification catalyst in a reaction zone under conditions which are effective to produce an intermediate product which has a reduced amount of olefinic unsaturation relative to that of the feedstock as measured by bromine number. The intermediate product is then separated into at least three fractions of different volatility, and the lowest boiling first fraction is contacted with a hydrodesulfurization catalyst in the presence of hydrogen under conditions which are effective to convert at least a portion of its sulfur-containing impurities to hydrogen sulfide. The intermediate boiling fraction is contacted with a selective hydrotreating catalyst in the presence of hydrogen under conditions which are effective to convert at least a portion of its sulfur-containing impurities to hydrogen sulfide.

[0001] The present application claims the benefit of U.S. provisionalapplication No. 60/334,640 filed on Oct. 25, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to a process for removingsulfur-containing impurities from olefin-containing hydrocarbonmixtures. More particularly, the process involves converting thefeedstock to an intermediate product of reduced bromine number,separating the intermediate product into fractions of different boilingpoint, and subjecting the high boiling fraction to hydrodesulfurization,and the intermediate boiling fraction to selective hydrotreating.

BACKGROUND OF THE INVENTION

[0003] The fluidized catalytic cracking process is one of the majorrefining processes which is currently employed in the conversion ofpetroleum to desirable fuels such as gasoline and diesel fuel. In thisprocess, a high molecular weight hydrocarbon feedstock is converted tolower molecular weight products through contact with hot,finely-divided, solid catalyst particles in a fluidized or dispersedstate. Suitable hydrocarbon feedstocks typically boil within the rangefrom about 205° C. to about 650° C., and they are usually contacted withthe catalyst at temperatures in the range from about 450° C. to about650° C. Suitable feedstocks include various mineral oil fractions suchas light gas oils, heavy gas oils, wide-cut gas oils, vacuum gas oils,kerosenes, decanted oils, residual fractions, reduced crude oils andcycle oils which are derived from any of these as well as fractionsderived from shale oils, tar sands processing, and coal liquefaction.Products from a fluidized catalytic cracking process are typically basedon boiling point and include light naphtha (boiling between about 10° C.and about 221° C.), heavy naphtha (boiling between about 10° C. andabout 249° C.), kerosene (boiling between about 180° C. and about 300°C.), light cycle oil (boiling between about 221° C. and about 345° C.),and heavy cycle oil (boiling at temperatures higher than about 345° C.).

[0004] Naphtha from a catalytic cracking process comprises a complexblend of hydrocarbons which includes paraffins (also known as alkanes),cycloparaffins (also known as cycloalkanes or naphthenes), olefins (asused herein, the term olefin includes all acyclic and cyclichydrocarbons which contain at least one double bond and are notaromatic), and aromatic compounds. Such a material typically contains arelatively high olefin content and includes significant amounts ofsulfur-containing aromatic compounds, such as thiophenic andbenzothiophenic compounds, as impurities. For example, a light naphthafrom the fluidized catalytic cracking of a petroleum derived gas oil cancontain up to about 60 wt. % of olefins and up to about 0.7 wt. % ofsulfur wherein most of the sulfur will be in the form of thiophenic andbenzothiophenic compounds. However, a typical naphtha from the catalyticcracking process will usually contain from about 5 wt. % to about 40 wt.% olefins and from about 0.07 wt. % to about 0.5 wt. % sulfur.

[0005] Not only does the fluidized catalytic cracking process provide asignificant part of the gasoline pool in the United States, it alsoprovides a large proportion of the sulfur that appears in this pool. Thesulfur in the liquid products from this process is in the form oforganic sulfur compounds and is an undesirable impurity which isconverted to sulfur oxides when these products are utilized as a fuel.The sulfur oxides are objectionable air pollutants. In addition, theycan deactivate many of the catalysts that have been developed for thecatalytic converters which are used on automobiles to catalyze theconversion of harmful engine exhaust emissions to gases which are lessobjectionable. Accordingly, it is desirable to reduce the sulfur contentof catalytic cracking products to the lowest possible levels.

[0006] Low sulfur products are conventionally obtained from thecatalytic cracking process by hydrotreating either the feedstock to theprocess or the products from the process. The hydrotreating processinvolves treatment of the feedstock with hydrogen in the presence of acatalyst and results in the conversion of the sulfur in thesulfur-containing impurities to hydrogen sulfide, which can be separatedand converted to elemental sulfur. The hydrotreating process can resultin the destruction of olefins in the feedstock by converting them tosaturated hydrocarbons through hydrogenation. This destruction ofolefins by hydrogenation is usually undesirable because: (1) it resultsin the consumption of expensive hydrogen, and (2) the olefins areusually valuable as high octane components of gasoline. As an example, atypical naphtha of gasoline boiling range from a catalytic crackingprocess has a relatively high octane number as a result of a largeolefin content. Hydrotreating such a material causes a reduction in theolefin content in addition to the desired desulfurization, and theoctane number of the hydrotreated product decreases as the degree orseverity of the desulfurization increases.

[0007] U.S. Pat. No. 5,865,988 (Collins et al.) is directed to a twostep process for the production of low sulfur gasoline from an olefinic,cracked, sulfur-containing naphtha. The process involves: (a) passingthe naphtha over a shape selective acidic catalyst, such as ZSM-5zeolite, to selectively crack low octane paraffins and to convert someof the olefins and naphthenes to aromatics and aromatic side chains; and(2) hydrodesulfurizing the resulting product over a hydrotreatingcatalyst in the presence of hydrogen. It is disclosed that the initialtreatment with the shape selective acidic catalyst removes the olefinswhich would otherwise be saturated in the hydrodesulfurization step.

[0008] International Patent Application No. WO 98/30655 (Huff et al.),published under the Patent Cooperation Treaty, discloses a process forthe production of a product of reduced sulfur content from a feedstockwherein the feedstock is comprised of a mixture of hydrocarbons andcontains organic sulfur compounds as unwanted impurities. This processinvolves converting at least a portion of the sulfur-containingimpurities to sulfur-containing products of a higher boiling point bytreatment with an alkylating agent in the presence of an acid catalystand removing at least a portion of these higher boiling products byfractionation on the basis of boiling point.

[0009] U.S. Pat. Nos. 5,298,150 (Fletcher et al.); 5,346,609 (Fletcheret al.); 5,391,288 (Collins et al.); and 5,409,596 (Fletcher et al.) areall directed to a two step process for the preparation of a low sulfurgasoline wherein a naphtha feedstock is subjected tohydrodesulfurization followed by treatment with a shape selectivecatalyst to restore the octane which is lost during thehydrodesulfurization step.

[0010] U.S. Pat. No. 5,171,916 (Le et al.) is directed to a process forupgrading a light cycle oil by: (1) alkylating the heteroatom containingaromatics of the cycle oil with an aliphatic hydrocarbon having at leastone olefinic double bond through the use of a crystallinemetallosilicate catalyst; and (2) separating the high boiling alkylationproduct by fractional distillation. It is disclosed that the unconvertedlight cycle oil has a reduced sulfur and nitrogen content, and the highboiling alkylation product is useful as a synthetic alkylated aromaticfunctional fluid base stock.

[0011] U.S. Pat. No. 5,599,441 (Collins et al.) discloses a process forremoving thiophenic sulfur compounds from a cracked naphtha by: (1)contacting the naphtha with an acid catalyst in an alkylation zone toalkylate the thiophenic compounds using the olefins present in thenaphtha as an alkylating agent; (2) removing an effluent stream from thealkylation zone; and (3) separating the alkylated thiophenic compoundsfrom the alkylation zone effluent stream by fractional distillation. Itis also disclosed that the sulfur-rich high boiling fraction from thefractional distillation may be desulfurized using conventionalhydrotreating or other desulfurization processes.

[0012] U.S. Pat. No. 5,863,419 (Huff, Jr. et al.) discloses a catalyticdistillation process for the production of a product of reduced sulfurcontent from a feedstock wherein the feedstock is comprised of a mixtureof hydrocarbons which contains organic sulfur compounds as unwantedimpurities. The process involves carrying out the following processsteps simultaneously within a distillation column reactor: (1)converting at least a portion of the sulfur-containing impurities tosulfur-containing products of a higher boiling point by treatment withan alkylating agent in the presence of an acid catalyst; and (2)removing at least a portion of these higher boiling products byfractional distillation. It is also disclosed that the sulfur-rich highboiling fraction can be efficiently hydrotreated at relatively low costbecause of its reduced volume relative to that of the originalfeedstock.

[0013] More recently, U.S. Pat. No. 6,024,865 in the name of Bruce D.Alexander, George A. Huff, Vivek R. Pradhan, William J. Reagan and RogerH. Cayton disclosed a product of reduced sulfur content which isproduced from a feedstock which is comprised of a mixture ofhydrocarbons and includes sulfur-containing aromatic compounds asunwanted impurities. The process involves separating the feedstock byfractional distillation into a lower boiling fraction which contains themore volatile sulfur-containing aromatic impurities and at least onehigher boiling fraction which contains the less volatilesulfur-containing aromatic impurities. Each fraction is then separatelysubjected to reaction conditions which are effective to convert at leasta portion of its content of sulfur-containing aromatic impurities tohigher boiling sulfur-containing products by alkylation with analkylating agent in the presence of an acidic catalyst. The higherboiling sulfur-containing products are removed by fractionaldistillation. It is also stated that alkylation can be achieved instages with the proviso that the conditions of alkylation are lesssevere in the initial alkylation stage than in a secondary stage, e.g.,through the use of a lower temperature in the first stage as opposed toa higher temperature in a secondary stage.

[0014] U.S. Pat. No. 6,059,962 in the name of Bruce D. Alexander, GeorgeA. Huff, Vivek R. Pradhan, William J. Reagan and Roger H. Claytondisclosed a product of reduced sulfur content wherein the product isproduced in a multiple stage process from a feedstock which is comprisedof a mixture of hydrocarbons and includes sulfur-containing aromaticcompounds as unwanted impurities. The first stage involves: (1)subjecting the feedstock to alkylation conditions which are effective toconvert a portion of the impurities to higher boiling sulfur-containingproducts, and (2) separating the resulting products by fractionaldistillation into a lower boiling fraction and a higher boilingfraction. The lower boiling fraction is comprised of hydrocarbons and isof reduced sulfur content relative to the feedstock. The higher boilingfraction is comprised of hydrocarbons and contains unconvertedsulfur-containing aromatic impurities and also the higher boilingsulfur-containing products. Each subsequent stage involves: (1)subjecting the higher boiling fraction from the preceding stage toalkylation conditions which are effective to convert at least a portionof its content of sulfur-containing aromatic compounds to higher boilingsulfur-containing products, and (2) separating the resulting products byfractional distillation into a lower boiling hydrocarbon fraction and ahigher boiling fraction which contains higher boiling sulfur-containingalkylation products. The total hydrocarbon product of reduced sulfurcontent from the process is comprised of the lower boiling fractionsfrom various stages.

[0015] Another approach to reducing the sulfur-containing organicimpurities content of a feedstock comprised of a normally liquid mixtureof hydrocarbons which includes olefins is disclosed in InternationalPublication Number WO 01/53432, A1. This approach involves (a)contacting the feedstock with an olefin-modification catalyst in anolefin-modification reaction zone under conditions which are effectiveto produce a product having a bromine number which is lower than that ofthe feedstock; (b) fractionating the product from theolefin-modification reaction zone to produce: (i) a first fraction whichcomprises sulfur-containing organic impurities and has a distillationendpoint which is in the range from about 135° C. to about 221° C.; and(ii) a second fraction which is higher boiling than the first fractionand comprises sulfur-containing organic impurities; and (c) contactingthe first fraction with a hydrodesulfurization catalyst in the presenceof hydrogen in a hydrodesulfurization reaction zone under conditionswhich are effective to convert at least a portion of the sulfur in thesulfur-containing impurities of the first fraction to hydrogen sulfide.

[0016] It has now been discovered that the sulfur-containing organicimpurities while undergoing reaction in the olefin-modification zoneform refractive sulfur compounds. The formation of these refractivecompounds is undesirable because they can only be treated byconventional hydrodesulfurization means which results in undesirableconcomitant octane loss. These refractive sulfur compounds cannot beremoved via a selective hydrotreating process. It has further beendiscovered that these refractive sulfur compounds concentrate in thefraction of the olefin-modification reaction zone product having aboiling range above about 200° C. The removal of these compounds canonly be achieved through conventional hydrodesulfurization that resultsin the saturation of olefins and the loss of octane. Trace amounts ofolefin-modification catalyst are also leached off of the catalyst in theolefin modification reaction zone and pass into the olefin-modificationzone product. There is a potential for these compounds or componentscontaining the leached olefin-modification catalyst to cause bothcatalyst deactivation and pressure drop in any downstream units such asdownstream hydrotreaters. Additionally it has been discovered that thesecompounds like the refractive sulfur compounds tend to concentrate inthe 200° C. plus boiling range fraction.

[0017] By utilizing the process of the present invention the problemsassociated with refractive sulfur compounds and compounds or componentscontaining leached olefin-modification catalyst can be ameliorated byfractionating the product from the olefin-modification zone into atleast three fractions. The advantage of splitting theolefin-modification zone product into at least three fractions is thatthe refractive compounds and leached catalyst containing compounds orcomponents can be recovered in a relatively small volume stream of thehighest boiling fraction of the olefin modification zone product. Theremainder of the olefin-modification zone product is split into at leasttwo fractions wherein the lowest boiling fraction is relativelydesulfurized and can be passed directly to the gasoline pool, and theintermediate boiling fraction can be passed to a selective hydrotreatingzone, wherein the octane number is retained while the sulfur-containingorganic impurities are converted to hydrogen sulfide. Although notpreferred alternatively, the intermediate fraction can be passed to aconventional hydrotreater and subsequently to a reformer to upgrade theoctane of this fraction. In accordance with the process of the inventionthe bulk of the olefin-modification zone effluent, e.g. 90 vol. % to 98vol. %, can be split into two fractions containing a paucity ofrefractive sulfur and leached olefin modification catalyst and not besubjected to an octane reducing hydrodesulfurization step.

[0018] The highest boiling fraction, ideally, can be routed to aconventional diesel or naphtha hydrotreater or back to the fluidizedcatalytic cracking unit for removal of both the refractive andnon-refractive sulfur compounds. The leached olefin-modificationcompounds or components can be removed with activated aluminum viaconventional means known to those skilled in the art prior to beingpassed to the hydrotreater.

SUMMARY OF THE INVENTION

[0019] Hydrocarbon liquids which boil at standard pressure over either abroad or a narrow range of temperatures within the range from about 10°C. to about 345° C. are referred to herein as “hydrocarbon liquids.”Such liquids are frequently encountered in the refining of petroleum andalso in the refining of products from coal liquefaction and theprocessing of oil shale or tar sands, and these liquids are typicallycomprised of a complex mixture of hydrocarbons, and these mixtures caninclude paraffins, cycloparaffins, olefins and aromatics. For example,light naphtha, heavy naphtha, gasoline, kerosene and light cycle oil areall hydrocarbon liquids.

[0020] Hydrocarbon liquids which are encountered in a refineryfrequently contain undesirable sulfur-containing impurities which mustbe at least partially removed. Hydrotreating procedures are effectiveand are commonly used for removing sulfur-containing impurities fromhydrocarbon liquids. Unfortunately, conventional hydrotreating processesare usually unsatisfactory for use with highly olefinic hydrocarbonliquids because such processes result in significant conversion of theolefins to paraffins which are usually of lower octane. In addition, thehydrogenation of the olefins results in the consumption of expensivehydrogen.

[0021] In accordance with International Publication Number WO 01/53432,A1 organic sulfur compounds can be removed from hydrocarbon liquids by amultiple step process which comprises (a) contacting the feedstock withan olefin-modification reaction zone under conditions which areeffective to produce a product having a bromine number which is lowerthan that of the feedstock;(b) fractionating the product into twofractions: namely a first fraction having a distillation endpoint of 135C. to 221 C. and a higher boiling fraction; and (c) carrying out ahydrodesulfurization reaction with the lower boiling fraction.

[0022] Unfortunately such a process results in undesirable octane lossbecause the first fraction is subjected to conventional hydrotreatingwhich serves to reduce octane via olefin saturation. Additionally such aprocess does not address or present a solution to the problemsassociated with converting refractive sulfur compounds to hydrogensulfide while retaining octane. Further, such a process does not addressthe difficulties presented downstream of the olefin-modification zone bythe leached catalyst compounds or components such as increased pressuredrop and catalyst deactivation in downstream units.

[0023] Accordingly, there is a need for a process which can achieve asubstantially complete removal of sulfur-containing impurities fromolefin-containing hydrocarbon liquids which: (1) is relativelyinexpensive to carry out, (2) results in little if any octane loss; and(3) addresses the problems associated with refractive sulfur compoundsand leached catalyst compounds or components. For example, there is aneed for such a process which can be used to remove sulfur-containingimpurities from hydrocarbon liquids, such as products from a fluidizedcatalytic cracking process, which are highly olefinic and containrelatively large amounts of sulfur-containing organic materials such asmercaptans, thiophenic compounds, and benzothiophenic compounds asunwanted impurities.

[0024] It has now been discovered that such an improved process involvesmodifying the olefin content of the feedstock over anolefin-modification catalyst in an olefin-modification step,fractionating the products from the olefin-modification step into atleast three fractions on the basis of boiling point, selectivelyhydrotreating the intermediate boiling fraction, and hydrodesulfurizingthe highest boiling of the resulting fractions. The olefin-modificationstep results in a reduction of the olefinic unsaturation of thefeedstock, as measured by bromine number. As a consequence of theolefin-modification step, a sulfur-lean product is obtained from thesubsequent selective hydrotreating step which has little loss of octanerelative to that of the feedstock to the olefin-modification step. Inaddition, the reduction of olefinic unsaturation in theolefin-modification step results in a corresponding reduction ofhydrogen consumption in the respective selective hydrotreating andhydrodesulfurization steps since there is a reduced number of olefinicdouble bonds to consume hydrogen in hydrogenation reactions.

[0025] One embodiment of the invention is a process for producing aproduct of reduced sulfur content from a feedstock, wherein saidfeedstock contains sulfur-containing organic impurities and is comprisedof a normally liquid mixture of hydrocarbons which includes olefins,said process comprising:

[0026] (a) contacting the feedstock with an olefin-modification catalystin an olefin-modification reaction zone under conditions which areeffective to produce a product having a bromine number which is lowerthan that of the feedstock;

[0027] (b) fractionating the product from said olefin-modificationreaction zone to produce:

[0028] (i) a first fraction which comprises sulfur-containing organicimpurities and has a distillation endpoint which is less than about 140°C.;

[0029] (ii) a second fraction which is higher boiling than the firstfraction which comprises sulfur-containing organic impurities and has adistillation endpoint which is less than about 240° C.; and

[0030] (iii) a third fraction which is higher boiling than the secondfraction and comprises sulfur-containing organic impurities andrefractive sulfur compounds;

[0031] (c) contacting the second fraction with a selective hydrotreatingcatalyst in the presence of hydrogen in a selective hydrotreatingreaction zone under conditions which are effective to convert at least aportion of the sulfur in said sulfur-containing impurities of the secondfraction to hydrogen sulfide; and

[0032] (d) contacting the third fraction with a hydrodesulfurizationcatalyst in the presence of hydrogen in a hydrodesulfurization reactionzone under conditions which are effective to convert at least a portionof the sulfur in said sulfur-containing impurities of the third fractionto hydrogen sulfide.

[0033] Another embodiment of the invention is a process for producingproducts of reduced sulfur content from a feedstock, wherein saidfeedstock contains sulfur-containing organic impurities and is comprisedof a normally liquid mixture of hydrocarbons which includes olefins,said process comprising:

[0034] (a) contacting the feedstock with an olefin-modification catalystin an olefin-modification reaction zone under conditions which areeffective to produce a product which has a lower bromine number thanthat of the feedstock, wherein said olefin-modification catalyst is anacid catalyst;

[0035] (b) fractionating the product from said olefin-modificationreaction zone to produce:

[0036] (i) a first fraction which comprises sulfur-containing organicimpurities and has a distillation endpoint which is less than about 120°C.;

[0037] (ii) a second fraction which is higher boiling than the firstfraction which comprises sulfur-containing organic impurities and has adistillation endpoint which is less than about 200° C.; and;

[0038] (iii) a third fraction which is higher boiling than the secondfraction and comprises sulfur-containing organic impurities andrefractive sulfur compounds;

[0039] (c) contacting the second fraction with a selective hydrotreatingcatalyst in the presence of hydrogen in a selective hydrotreatingreaction zone under conditions which are effective to convert at least aportion of the sulfur in said sulfur-containing impurities of the secondfraction to hydrogen sulfide; and

[0040] (d) contacting the third fraction with a hydrodesulfurizationcatalyst in the presence of hydrogen in a hydrodesulfurization reactionzone under conditions which are effective to convert at least a portionof the sulfur in said sulfur-containing impurities of the third fractionto hydrogen sulfide.

[0041] In another embodiment although not preferred, the second fractioncan be passed to a hydrodesulfurization zone followed by a reformingzone to increase the octane number of the fraction which was reduced inthe hydrodesulfurization zone.

[0042] An object of the invention is to provide an improved process forthe removal of sulfur-containing impurities from a hydrocarbon liquidwhich contains a significant olefin content.

[0043] Another object of the invention is to provide an improved methodfor the efficient removal of sulfur-containing impurities from anolefinic cracked naphtha.

[0044] A further object of the invention is to provide an improvedmethod for desulfurizing an olefinic cracked naphtha which yields aproduct of substantially unchanged octane.

[0045] Yet another object of the invention is to provide an improvedmethod for handling problems associated with refractive sulfur compoundsand leached catalyst compounds or components in a process for removingsulfur-containing impurities in a hydrocarbon liquid.

BRIEF DESCRIPTION OF THE DRAWING

[0046] The drawing is a schematic representation of an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0047] A process for the production of a product of reduced sulfurcontent from an olefin-containing distillate hydrocarbon liquid whichcontains sulfur-containing impurities has been discovered. The processcan be used to produce a product which is substantially free ofsulfur-containing impurities, has a reduced olefin content, and has anoctane which is similar to that of the feedstock.

[0048] The invention involves contacting the feedstock with anolefin-modification catalyst in a reaction zone under conditions whichare effective to produce an intermediate product which has a reducedamount of olefinic unsaturation relative to that of the feedstock asmeasured by bromine number. The intermediate product is then separatedinto at least three fractions of different volatility. The fraction ofhighest volatility (i.e., the lowest boiling fraction) is relativelyfree of sulfur-containing organic impurities i.e. generally below 20parts per million by weight sulfur and therefore can be passed directlyto the gasoline pool. The second fraction or intermediate boiling rangefraction, is contacted with a selective hydrotreating catalyst in thepresence of hydrogen under conditions effective to convert at least aportion of its sulfur-containing organic impurities to hydrogen sulfide.The hydrogen sulfide can be easily removed by conventional methods toprovide a product of substantially reduced sulfur content relative tothe feedstocks. The third fraction or highest boiling fraction iscontacted with a hydrodesulfurization catalyst in the presence ofhydrogen under conditions which are effective to convert at least aportion of the sulfur-containing organic impurities and refractivesulfur compounds to hydrogen sulfide. The hydrogen sulfide can be easilyremoved by conventional methods to provide a product of substantiallyreduced sulfur content relative to the feedstocks.

[0049] Aromatic sulfur-containing impurities in the feedstock, such asthiophenic and benzothiophenic compounds, undergo conversion, at leastin part, within the olefin-modification reaction zone to higher boilingsulfur-containing products some of which can be characterized asrefractive sulfur compounds as further defined hereinafter. Thisconversion is believed to be a result of alkylation of the aromaticsulfur-containing impurities by olefins which is catalyzed by theolefin-modification catalyst. Upon fractionation of the effluent fromthe olefin-modification reaction zone, most of these high boilingsulfur-containing materials including the refractive sulfur compoundsappear in the third boiling fraction or highest boiling fraction, andthe first or lowest boiling fraction and the intermediate boilingfraction have a reduced sulfur content relative to that of the feedstockto the olefin-modification zone.

[0050] In a highly preferred embodiment, the second fraction iscontacted with a selective hydrotreating catalyst in the presence ofhydrogen under conditions which are effective to convert at least aportion of the fraction's sulfur-containing impurities to hydrogensulfide. This can be accomplished by utilizing one of the selectivehydrotreating processes that are presently being licensed such as theSCANfining process licensed by ExxonMobil Research and EngineeringCompany and the PRIME-G+ process licensed by IFP North America Inc. orby operating a conventional hydrotreating process at selectivehydrotreating conditions that are relatively less severe such that thedesulfurization occurs while limiting olefin saturation. Such selectivehydrotreating processes are also disclosed in U.S. Pat. No. 6,007,704(Chapus et al.), U.S. Pat. No. 5,821,397 (Joly et al.), and U.S. Pat.No. 6,255,548 (Didillon et al.)

[0051] The third fraction is contacted with a hydrodesulfurizationcatalyst in the presence of hydrogen under conditions which areeffective to convert at least a portion of its sulfur-containingimpurities including refractive sulfur compounds to hydrogen sulfide. Alarge portion of the sulfur-containing impurities of the higher boilingfraction or fractions will frequently be comprised of aromaticsulfur-containing compounds, such as thiophenic and benzothiopheniccompounds and refractive sulfur compounds, which are more difficult toremove by hydrodesulfurization than mercaptans and thiophenic compounds.Accordingly, a preferred embodiment of the invention will comprise theuse of more vigorous hydrodesulfurization conditions.

[0052] Feedstocks which can be used in the practice of this inventionare comprised of normally liquid hydrocarbon mixtures which containolefins and boil over a range of temperatures within the range fromabout 10° C. to about 345° C. as measured by the ASTM D 2887-97aprocedure (which can be found in the 1999 Annual Book of ASTM Standards,Section 5, Petroleum Products, Lubricants, and Fossil Fuels, Vol. 05.02,page 200, and said procedure is hereby incorporated herein by referencein its entirety) or by conventional alternative procedures. In addition,suitable feedstocks will preferably include a mixture of hydrocarbonswhich boils in the gasoline range. If desired, such feedstocks can alsocontain significant amounts of lower volatility hydrocarbon componentswhich have a higher boiling point than said high volatility fraction.The feedstock will be comprised of a normally liquid mixture ofhydrocarbons which desirably has a distillation endpoint which is about345° C. or lower, and is preferably about 249° C. or lower. Preferably,the feedstock will have an initial boiling point which is below about79° C. and a distillation endpoint which is not greater than about 345°C. Suitable feedstocks include any of the various complex mixtures ofhydrocarbons which are conventionally encountered in the refining ofpetroleum, such as natural gas liquids, naphthas, light gas oils, heavygas oils, and wide-cut gas oils, as well as hydrocarbon fractions whichare derived from coal liquefaction and the processing of oil shale ortar sands. Preferred feedstocks are comprised of olefin-containinghydrocarbon mixtures which are derived from the catalytic cracking orthe coking of hydrocarbon feedstocks.

[0053] Catalytic cracking products are highly preferred as a source offeedstock hydrocarbons for use in the subject invention. Materials ofthis type include liquids which boil below about 345° C., such as lightnaphtha, heavy naphtha and light cycle oil. However, it will also beappreciated that the entire output of volatile products from a catalyticcracking process can be utilized as a source of feedstock hydrocarbonsfor use in the practice of this invention. Catalytic cracking productsare a desirable source of feedstock hydrocarbons because they typicallyhave a relatively high olefin content and they usually containsubstantial amounts of organic sulfur compounds as impurities. Forexample, a light naphtha from the fluidized catalytic cracking of apetroleum derived gas oil can contain up to about 60 wt. % of olefinsand up to about 0.7 wt. % of sulfur wherein most of the sulfur will bein the form of thiophenic and benzothiophenic compounds. In addition,the sulfur-containing impurities will usually include mercaptans andorganic sulfides. A preferred feedstock for use in the practice of thisinvention will be comprised of catalytic cracking products and willcontain at least 1 wt. % of olefins. A preferred feedstock will becomprised of hydrocarbons from a catalytic cracking process and willcontain at least 10 wt. % of olefins. A highly preferred feedstock willbe comprised of hydrocarbons from a catalytic cracking process and willcontain at least about 15 wt. % or 20 wt. % of olefins.

[0054] In one embodiment of the invention, the feedstock for theinvention will be comprised of a mixture of low molecular weight olefinswith hydrocarbons from a catalytic cracking process. For example, afeedstock can be prepared by adding olefins which contain from 3 to 5carbon atoms to a naphtha from a catalytic cracking process.

[0055] In another embodiment of the invention, the feedstock for theinvention will be comprised of a mixture of a naphtha from a catalyticcracking process with a source of volatile aromatic compounds, such asbenzene and toluene. For example, a feedstock can be prepared by mixinga light reformate with a naphtha from a catalytic cracking process. Atypical light reformate will contain from about 0 to about 2 vol. %olefins, from about 20 to about 45 vol. % aromatics, and will havedistillation properties such that the 10% distillation point (“T10”) isno greater than about 160° F. (71° C.), the 50% distillation point(“T50”) is no greater than about 200° F. (93° C.), and the 90%distillation point (“T90”) is no greater than about 250° F. (121° C.).It will be understood that these distillation points refer to adistillation point obtained by the ASTM D 86-97 procedure (which can befound in the 1999 Annual Book of ASTM Standards, Section 5, PetroleumProducts, Lubricants, and Fossil Fuels, Vol. 05.01, page 16, and saidprocedure is hereby incorporated herein by reference in its entirety) orby conventional alternative procedures. A typical light reformate willcontain from about 5 to about 15 vol. % of benzene.

[0056] Another embodiment of the invention involves the use of afeedstock which is comprised of a mixture of: (1) hydrocarbons from acatalytic cracking process; (2) a source of volatile aromatic compounds;and (3) a source of olefins which contain from 3 to 5 carbon atoms.

[0057] Suitable feedstocks for the invention will contain at least 1 wt.% of olefins, preferably at least 10 wt. % of olefins, and morepreferably at least about 15 wt. % or 20 wt. % of olefins. If desired,the feedstock can have an olefin content of 50 wt. % or more. Inaddition, suitable feedstocks can contain from about 0.005 wt. % up toabout 2.0 wt. % of sulfur in the form of organic sulfur compounds.However, typical feedstocks will generally contain from about 0.05 wt. %up to about 0.7 wt. % sulfur in the form of organic sulfur compounds.

[0058] Feedstocks which are useful in the practice of this invention,such as naphtha from a catalytic cracking process, will occasionallycontain nitrogen-containing organic compounds as impurities in additionto the sulfur-containing impurities. Many of the typicalnitrogen-containing impurities are organic bases and, in some instances,can cause a relatively rapid deactivation of the olefin-modificationcatalyst of the subject invention. In the event that such deactivationis observed, it can be prevented by removal of the basicnitrogen-containing impurities before they can contact theolefin-modification catalyst. Accordingly, when the feedstock containsbasic nitrogen-containing impurities, a preferred embodiment of theinvention comprises removing these basic nitrogen-containing impuritiesfrom the feedstock before it is contacted with the olefin-modificationcatalyst. In another embodiment of the invention, a feedstock is usedwhich is substantially free of basic nitrogen-containing impurities (forexample, such a feedstock will contain less than about 50 ppm by weightof basic nitrogen). A highly desirable feedstock is comprised of atreated naphtha which is prepared by removing basic nitrogen-containingimpurities from a naphtha produced by a catalytic cracking process.

[0059] Basic nitrogen-containing impurities can be removed from thefeedstock or from a material that is to be used as a feedstock componentby any conventional method. Such methods typically involve treatmentwith an acidic material, and conventional methods include proceduressuch as washing with an aqueous solution of an acid or passing thematerial through a guard bed. In addition, a combination of suchprocedures can be used. Guard beds can be comprised of materials whichinclude but are not limited to A-zeolite, Y-zeolite, L-zeolite,mordenite, fluorided alumina, fresh cracking catalyst, equilibriumcracking catalyst and acidic polymeric resins. If a guard bed techniqueis employed, it is often desirable to use two guard beds in such amanner that one guard bed can be regenerated while the other is inservice. If a cracking catalyst is utilized to remove basicnitrogen-containing impurities, such a material can be regenerated inthe regenerator of a catalytic cracking unit when it has becomedeactivated with respect to its ability to remove such impurities. If anacid wash is used to remove basic nitrogen-containing compounds, thetreatment will be carried out with an aqueous solution of a suitableacid. Suitable acids for such use include but are not limited tohydrochloric acid, sulfuric acid and acetic acid. The concentration ofacid in the aqueous solution is not critical, but is conveniently chosento be in the range from about 0.1 wt. % to about 30 wt. %. For example,a 5 wt. % solution of sulfuric acid in water can be used to remove basicnitrogen containing impurities from a heavy naphtha produced by acatalytic cracking process.

[0060] The process of this invention is highly effective in removingsulfur-containing organic impurities of all types from the feedstock.Such impurities will typically include aromatic, sulfur-containing,organic compounds which include all aromatic organic compounds whichcontain at least one sulfur atom. Such materials include thiophenic andbenzothiophenic compounds, and examples of such materials include butare not limited to thiophene, 2-methylthiophene, 3-methylthiophene,2,3-dimethylthiophene, 2,5-dimethylthiophene, 2-ethylthiophene,3-ethylthiophene, benzothiophene, 2-methylbenzothiophene,2,3-dimethylbenzothiophene, and 3-ethylbenzothiophene. Other typicalsulfur-containing impurities include mercaptans and organic sulfides anddisulfides.

[0061] The olefin-modification catalyst of the invention can becomprised of any material which is capable of catalyzing theoligomerization of olefins. Desirably, the olefin-modification catalystwill be comprised of a material which is also capable of catalyzing thealkylation of aromatic organic compounds by olefins. Conventionalalkylation catalysts are highly suitable for use as theolefin-modification catalyst of this invention because they typicallyhave the ability to catalyze both olefin oligomerization and thealkylation of aromatic organic compounds by olefins. Although liquidacids, such as sulfuric acid can be used, solid acidic catalysts areparticularly desirable, and such solid acidic catalysts include liquidacids which are supported on a solid substrate. The solid catalysts aregenerally preferred over liquid catalysts because of the ease with whichthe feed can be contacted with such a material. For example, the feedcan simply be passed through one or more fixed beds of solid particulatecatalyst at a suitable temperature. Alternatively, the feed can bepassed through an ebulated bed of solid particulate catalyst.

[0062] Olefin-modification catalysts which are suitable for use in thepractice of the invention can be comprised of materials such as acidicpolymeric resins, supported acids, and acidic inorganic oxides. Suitableacidic polymeric resins include the polymeric sulfonic acid resins whichare well-known in the art and are commercially available. Amberlyst® 35,a product produced by Rohm and Haas Co., is a typical example of such amaterial.

[0063] Supported acids which are useful as olefin-modification catalystsinclude but are not limited to Brönsted acids (examples includephosphoric acid, sulfuric acid, boric acid, HF, fluorosulfonic acid,trifluoromethanesulfonic acid, and dihydroxyfluoroboric acid) and Lewisacids (examples include BF₃, BCl₃, AlCl₃, AlBr₃, FeCl₂, FeCl₃, ZnCl₂,SbF₅, SbCl₅ and combinations of AlCl₃ and HCl) which are supported onsolids such as silica, alumina, silica-aluminas, zirconium oxide orclays. When supported liquid acids are employed, the supported catalystsare typically prepared by combining the desired liquid acid with thedesired support and drying. Supported catalysts which are prepared bycombining a phosphoric acid with a support are highly preferred and arereferred to herein as solid phosphoric acid catalysts. These catalystsare preferred because they are both highly effective and low in cost.U.S. Pat. No. 2,921,081 (Zimmerschied et al.), which is incorporatedherein by reference in its entirety, discloses the preparation of solidphosphoric acid catalysts by combining a zirconium compound selectedfrom the group consisting of zirconium oxide and the halides ofzirconium with an acid selected from the group consisting oforthophosphoric acid, pyrophosphoric acid and triphosphoric acid. U.S.Pat. No. 2,120,702 (Ipatieff et al.), which is incorporated herein byreference in its entirety, discloses the preparation of solid phosphoricacid catalysts by combining a phosphoric acid with a siliceous material.Finally, British Patent No. 863,539, which is incorporated herein byreference in its entirety, also discloses the preparation of a solidphosphoric acid catalyst by depositing a phosphoric acid on a solidsiliceous material such as diatomaceous earth or kieselguhr.

[0064] With respect to a solid phosphoric acid that is prepared bydepositing a phosphoric acid on kieselguhr, it is believed that thecatalyst contains: (1) one or more free phosphoric acids (such asorthophosphoric acid, pyrophosphoric acid and triphosphoric acid)supported on kieselguhr; and (2) silicon phosphates which are derivedfrom the chemical reaction of the acid or acids with the kieselguhr.While the anhydrous silicon phosphates are believed to be inactive as anolefin-modification catalyst, it is also believed that they can behydrolyzed to yield a mixture of orthophosphoric and polyphosphoricacids which is active as an olefin-modification catalyst. The precisecomposition of this mixture will depend upon the amount of water towhich the catalyst is exposed. In order to maintain a solid phosphoricacid alkylation catalyst at a satisfactory level of activity when it isused as an olefin-modification catalyst with a substantially anhydrousfeedstock, it is conventional practice to add a small amount of analcohol, such as isopropyl alcohol, to the feedstock to maintain thecatalyst at a satisfactory level of hydration. It is believed that thealcohol undergoes dehydration upon contact with the catalyst, and thatthe resulting water then acts to hydrate the catalyst. If the catalystcontains too little water, it tends to have a very high acidity whichcan lead to rapid deactivation as a consequence of coking and, inaddition, the catalyst will not possess a good physical integrity.Further hydration of the catalyst serves to reduce its acidity andreduces its tendency toward rapid deactivation through coke formation.However, excessive hydration of such a catalyst can cause the catalystto soften, physically agglomerate, and create high pressure drops infixed bed reactors. Accordingly, there is an optimum level of hydrationfor a solid phosphoric acid catalyst, and this level of hydration willbe a function of the reaction conditions. Although the invention is notto be so limited, with solid phosphoric acid catalysts, we have foundthat a water concentration in the feedstock which is in the range fromabut 50 to about 1,000 ppm by weight will generally maintain asatisfactory level of catalyst hydration. If desired, this water can beprovided in the form of an alcohol such as isopropyl alcohol which isbelieved to undergo dehydration upon contact with the catalyst.

[0065] Acidic inorganic oxides which are useful as olefin-modificationcatalysts include but are not limited to aluminas, silica-aluminas,natural and synthetic pillared clays, and natural and synthetic zeolitessuch as faujasites, mordenites, L, omega, X, Y, beta, and ZSM zeolites.Highly suitable zeolites include beta, Y, ZSM-3, ZSM-4, ZSM-5, ZSM-18,and ZSM-20. If desired, the zeolites can be incorporated into aninorganic oxide matrix material such as a silica-alumina.

[0066] Olefin-modification catalysts can comprise mixtures of differentmaterials, such as a Lewis acid (examples include BF₃, BCl₃, SbF₅ andAlCl₃), a nonzeolitic solid inorganic oxide (such as silica, alumina andsilica-alumina), and a large-pore crystalline molecular sieve (examplesinclude zeolites, pillared clays and aluminophosphates).

[0067] In the event that a solid olefin-modification catalyst is used,it will desirably be in a physical form which will permit a rapid andeffective contacting with feed in the olefin-modification reaction zone.Although the invention is not to be so limited, it is preferred that asolid catalyst be in particulate form wherein the largest dimension ofthe particles has an average value which is in the range from about 0.1mm to about 2 cm. For example, substantially spherical beads of catalystcan be used which have an average diameter from about 0.1 mm to about 2cm. Alternatively, the catalyst can be used in the form of rods whichhave a diameter in the range from about 0.1 mm to about 1 cm and alength in the range from about 0.2 mm to about 2 cm.

[0068] In the practice of the invention, the feedstock is contacted withan olefin-modification catalyst in an olefin-modification reaction zoneunder conditions which are effective to produce a product having abromine number which is lower than that of the feedstock without causingany significant cracking of any paraffins in the feedstock. It will beunderstood that the “bromine number” referred to herein is preferablydetermined by the ASTM D 1159-98 procedure, which can be found in the1999 Annual Book of ASTM Standards, Section 5, Petroleum Products,Lubricants, and Fossil Fuels, Vol. 05.01, page 407, and said procedureis hereby incorporated herein by reference in its entirety. However,other conventional analytical procedures for the determination ofbromine number can also be used. The bromine number of the product fromthe olefin-modification reaction zone will desirably be no greater than80% that of the feedstock to said reaction zone, preferably no greaterthan 70% that of said feedstock, and more preferably no greater than 65%that of said feedstock.

[0069] The reaction zone can consist of one or more fixed bed reactorscontaining the same or different catalysts. A fixed reactor can alsocomprise a plurality of catalyst beds. The plurality of catalyst beds ina single fixed bed reactor can also comprise the same or differentcatalysts.

[0070] The conditions utilized in the olefin-modification reaction zoneare also preferably selected so that at least a portion of the olefinsin the feedstock is converted to products which are of a suitablevolatility to be useful as components of fuels, such as gasoline anddiesel fuels.

[0071] Although the invention is not to be so limited, it is believedthat the olefins in the feedstock to the olefin-modification reactionzone are at least partially consumed in a variety of chemical reactionsupon contact of the feedstock with the olefin-modification catalyst insaid zone. And it is believed that the specific chemical reactions willdepend upon the composition of the feedstock. These chemical processesare believed to include olefin polymerization and the alkylation ofaromatic compounds by olefins.

[0072] The condensation reaction of an olefin or a mixture of olefinsover an olefin-modification catalyst to form higher molecular weightproducts is referred to herein as a polymerization process, and theproducts can be either low molecular weight oligomers or high molecularweight polymers. Oligomers are formed by the condensation of 2, 3 or 4olefin molecules with each other, while polymers are formed by thecondensation of 5 or more olefin molecules with each other. As usedherein, the term “polymerization” is used to broadly refer to a processfor the formation of oligomers and/or polymers. Olefin polymerizationresults in a consumption of olefinic unsaturation. For example, thesimple condensation of two molecules of propene results in the formationof a six carbon olefin which has only a single olefinic double bond (2double bonds in the starting materials have been replaced by 1 doublebond in the product). Similarly, the simple condensation of threemolecules of propene results in the formation of a nine carbon olefinwhich has only a single olefinic double bond (3 double bonds in thestarting materials have been replaced by 1 double bond in the product).

[0073] Although olefin polymerization is a simple model forunderstanding the reduction in bromine number that occurs in theolefin-modification reaction zone, it is believed that other processesare also important. For example, the initial products of simple olefincondensation can undergo isomerization in the presence of theolefin-modification catalyst to yield highly branched monounsaturatedolefins. In addition, polymerization reactions may occur to yieldpolymers which subsequently undergo fragmentation in the presence of theolefin-modification catalyst to yield highly branched products which areof a lower molecular weight than the initial polymerization product.Although the invention is not to be so limited, it is believed that thefollowing transformations occur within the olefin-modification reactionzone: (1) olefins in the feedstock which are of low molecular weight areconverted to olefins of higher molecular weight which are both highlybranched and within the gasoline boiling range; and (2) unbranched ormodestly branched olefins in the feedstock are isomerized to highlybranched olefins which are within the gasoline boiling range.

[0074] The alkylation of aromatic compounds is also an importantchemical process which can occur in the olefin-modification reactionzone and acts to reduce the bromine number of the feedstock. Thealkylation of an aromatic organic compound by an olefin, which containsa single double bond, results in the destruction of the double bond ofthe olefin and results in the substitution of an alkyl group for ahydrogen atom on the aromatic ring system of the substrate. Thisdestruction of the olefinic double bond of the olefin contributes to theformation of a product in the olefin-modification reaction zone whichhas a reduced bromine number relative to that of the feedstock. However,aromatic organic compounds vary widely in their reactivity as alkylationsubstrates. For example, the relative reactivities of somerepresentative aromatic compounds toward alkylation by 1-heptene at 204°C. over a solid phosphoric acid catalyst are set forth in Table I,wherein each rate constant was derived from the slope of the lineobtained by plotting experimental data in the form of ln(1−x) as afunction of time where x is the substrate concentration.

[0075] As used herein, the term “sulfur-containing aromatic compound”and “sulfur-containing aromatic impurity” refer to any aromatic organiccompound which contains at least one sulfur atom in its aromatic ringsystem. Such materials include thiophenic and benzothiophenic compounds.

[0076] Sulfur-containing aromatic compounds are usually alkylated morerapidly than aromatic hydrocarbons. Accordingly, the sulfur-containingaromatic impurities can, to a limited degree, be selectively alkylatedin the olefin-modification reaction zone. However, if desired, thereaction conditions in the reaction zone can be selected so thatsignificant alkylation of aromatic hydrocarbons does take place. Thisembodiment of the invention can be very useful if the feedstock containsvolatile aromatic hydrocarbons, such as benzene, and it is desired todestroy such material by conversion to higher molecular weightalkylation products. This embodiment is particularly useful when thefeedstock contains significant amounts of low molecular weight olefins,such as olefins which contain from 3 to 5 carbon atoms. The productsfrom mono- or dialkylation of benzene with such low molecular weightolefins will contain from 9 to 16 carbon atoms and, accordingly, will beof sufficient volatility to be useful as components of gasoline ordiesel fuels. TABLE I Alkylation Rate Constants for Various AromaticSubstrates upon Reaction with 1-Heptene at 204° C. over a SolidPhosphoric Acid Catalyst. Compound Rate Constant, min⁻¹ Thiophene 0.0772-Methylthiophene 0.046 2,5-Dimethylthiophene 0.004 Benzothiophene 0.008Benzene 0.001 Toluene 0.002

[0077] The alkylation of sulfur-containing aromatic impurities in thefeedstock to the olefin-modification reaction zone results in theformation of higher boiling sulfur-containing products. Accordingly,such materials can be removed by fractionation of the reaction zoneeffluent on the basis of boiling point. As a very crude approximation,each carbon atom in the side chain of a monoalkylated thiophene addsabout 25° C. to the 84° C. boiling point of thiophene. As an example,2-octylthiophene has a boiling point of 259° C., which corresponds to aboiling point increase of 23° C. over that of thiophene for each carbonatom in the eight carbon alkyl group. Accordingly, monoalkylation ofthiophene with a C₇ to C₁₅ olefin in the olefin-modification reactionzone will usually yield a sulfur-containing alkylation product which hasa high enough boiling point to be easily removed by fractionaldistillation as a component of a high boiling fraction which has aninitial boiling point of about 210° C.

[0078] The alkylation of a sulfur-containing aromatic compound by anolefin is illustrated by the mono-alkylation of thiophene with propeneto yield either 2-isopropylthiophene or 3-isopropylthiophene. The highermolecular weight of such an alkylation product is reflected by a higherboiling point relative to that of the starting material. In oneembodiment of the invention, reaction conditions in theolefin-modification reaction zone are selected so that a major portionof any sulfur-containing aromatic impurities in the feedstock areconverted to higher boiling sulfur-containing products.

[0079] Mercaptans are a class of organic sulfur-containing compoundswhich frequently appear in significant quantity as impurities in thehydrocarbon liquids which are conventionally encountered in the refiningof petroleum. For example, straight run gasolines, which are prepared bysimple distillation of crude oil, will frequently contain significantamounts of mercaptans and sulfides as impurities. In addition,benzothiophenic compounds and some multisubstituted thiophenes, such ascertain 2,5-dialkylthiophenes, will also be relatively unreactive underthe conditions employed in the olefin-modification reaction zone.Accordingly, a large proportion of the mercaptans in the feedstock andsignificant amounts of certain relatively unreactive sulfur-containingaromatic compounds can survive the reaction conditions in theolefin-modification reaction zone.

[0080] In the practice of this invention, the feedstock is contactedwith the olefin-modification catalyst within the olefin-modificationreaction zone at a temperature and for a period of time which areeffective to result in the desired reduction of the feedstock's olefinicunsaturation as measured by bromine number. The contacting temperaturewill be desirably in excess of about 50° C., preferably in excess of100° C. and more preferably in excess of 125° C. The contacting willgenerally be carried out at a temperature in the range from about 50° C.to about 350° C., preferably from about 100° C. to about 350° C., andmore preferably from about 125° C. to about 250° C. It will beappreciated, of course, that the optimum temperature will be a functionof the olefin-modification catalyst used, the olefin concentration inthe feedstock, the type of olefins present in the feedstock, and thetype of aromatic compounds in the feedstock that are to be alkylated.

[0081] The feedstock can be contacted with the olefin-modificationcatalyst in the olefin-modification reaction zone at any suitablepressure. However, pressures in the range from about 0.01 to about 200atmospheres are desirable, and a pressure in the range from about 1 toabout 100 atmospheres is preferred. When the feedstock is simply allowedto flow through a catalyst bed, it is generally preferred to use apressure at which the feed will be a liquid.

[0082] In a highly preferred embodiment of the invention, the conditionsutilized in the olefin-modification reaction zone are selected so thatno significant cracking of paraffins in the feedstock takes place. Forexample, desirably less than 10% of the paraffins in the feedstock willbe cracked, preferably less than 5% of the paraffins will be cracked,and more preferably less than 1% of the paraffins will be cracked. It isbelieved that any significant cracking of paraffins will result in theformation of undesirable by-products, for example, the formation of lowmolecular weight compounds which results in gasoline volume loss.

[0083] In the practice of the invention, the effluent from theolefin-modification reaction zone is fractionated on the basis ofvolatility into at least three fractions.

[0084] The distillation initial boiling point of the highest boilingthird fraction is desirably greater than about 200° C.

[0085] This fraction will contain a concentration of various compoundsthat can cause rapid catalyst deactivation in downstream selective andconventional hydrotreaters. Specifically, in the olefin-modificationreaction zone, refractive sulfur species are created. These refractivesulfur compounds concentrate in the 200° C. plus fraction of theolefin-modification zone product. The removal of these compounds canonly be accomplished via conventional hydrotreating orhydrodesulfurization which also detrimentally results in the saturationof olefins causing octane loss.

[0086] It is believed these refractive sulfur compounds have thefollowing structure:

[0087] Where R2 and R1 must have two or more carbons, e.g. C2H5, C3H7etc. and one chain must have more than five carbons, e.g. C5H11, C6H13,etc.

[0088] Thus examples of refractive sulfur compounds are:

[0089] It is believed refractive sulfur is a thiophene containing sevenor more alkyl carbons.

[0090] Additionally, trace amounts of the olefin-modification catalystcan be leached off in the olefin-modification zone and enter theolefin-modification zone product. This leached catalyst can causecatalyst deactivation and/or pressure drop difficulties in anydownstream selective hydrotreater or hydrodesulfurization reactor. Thisleached catalyst tends to concentrate in the 200° C. plus boilingfraction.

[0091] Other undesirable compounds that concentrate in the 200° C. plusboiling range fraction include nitrogen-containing compounds and dienes.

[0092] The advantage of the process of the invention is that thesecompounds can be recovered in the relative small volume of the 200° C.plus fraction. The volume of this fraction can range from 2 volumepercent to 10 volume percent of the olefin-modification zone product.Preferably, this volume percent can range from 2 volume percent to 6volume percent.

[0093] Thus the bulk, e.g., 90-98 volume percent, of theolefin-modification product, can be split into two fractions. The 140°C. minus, or more preferably the 120 ° C. minus boiling range first orlowest boiling fraction can be routed directly to gasoline. The lowestboiling range first fraction is typically sulfur free such that itcontains less than about 50 parts per million by weight sulfur andpreferably less than 30 parts per million by weight sulfur and mostpreferably less than 20 parts per million by weight, and can thereforebe directly used as blending stock for gasoline.

[0094] The intermediate fraction or second boiling fraction, preferablyhas a distillation end point of less than about 240° C. and mostpreferably less than about 200° C. The intermediate or second boilingfraction is passed to a selective hydrotreating zone that removes sulfurcompounds while retaining the octane.

[0095] The highest boiling fraction, which has a boiling range higherthan the intermediate fraction, from fractionation of the product fromthe olefin-modification reaction zone is contacted with ahydrodesulfurization catalyst in the presence of hydrogen underconditions which are effective to convert at least a portion of thesulfur in its sulfur-containing organic impurities including therefractive sulfur compounds to hydrogen sulfide. In a highly preferredembodiment, at least a portion of the higher boiling fraction orfractions are also contacted with a hydrodesulfurization catalyst in thepresence of hydrogen under conditions which are effective to convert atleast a portion of the sulfur in its sulfur-containing organicimpurities to hydrogen sulfide. Alternatively the highest boilingfraction can be recycled to the fluidized catalytic cracking unit.

[0096] The hydrodesulfurization catalyst can be any conventionalcatalyst, for example, a catalyst comprised of a Group VI and/or a GroupVIII metal which is supported on a suitable substrate. The Group VImetal is typically molybdenum or tungsten, and the Group VIII metal istypically nickel or cobalt. Typical combinations include nickel withmolybdenum and cobalt with molybdenum. Suitable catalyst supportsinclude, but are not limited to, alumina, silica, titania, calciumoxide, magnesia, strontium oxide, barium oxide, carbon, zirconia,diatomaceous earth, and lanthanide oxides. Preferred catalyst supportsare porous and include alumina, silica, and silica-alumina.

[0097] The particle size and shape of the hydrodesulfurization catalystwill typically be determined by the manner in which the reactants arecontacted with the catalyst. For example, the catalyst can be used as afixed bed catalyst or as an ebulating bed catalyst.

[0098] The hydrodesulfurization reaction conditions used in the practiceof this invention are conventional in character. For example, thepressures can range from about 15 to about 1500 psi (about 1.02 to about102.1 atmospheres); the temperature can range from about 50° C. to about450° C., and the liquid hourly space velocity can range from about 0.5to about 15 LHSV. The ratio of hydrogen to hydrocarbon feed in thehydrodesulfurization reaction zone will typically range from about 200to about 5000 standard cubic feet per barrel. The extent ofhydrodesulfurization will be a function of the hydrodesulfurizationcatalyst and reaction conditions selected and also the precise nature ofthe sulfur-containing organic impurities in the feed to thehydrodesulfurization reaction zone. However, the hydrodesulfurizationprocess conditions will be desirably selected so that at least about 50%of the sulfur content of the sulfur-containing organic impurities isconverted to hydrogen sulfide, and preferably so that the conversion tohydrogen sulfide is at least about 75% or more.

[0099] After removal of hydrogen sulfide, the product fromhydrodesulfurization of the highest boiling fraction from theolefin-modification reaction zone will have a sulfur content which isdesirably less than 50 ppm by weight, preferably less than 30 ppm byweight, and more preferably less than 10 ppm by weight. The octane ofthis hydrodesulfurization product will be desirably at least 90% that ofthe feedstock to the olefin-modification reaction zone, preferably atleast 95% that of said feedstock, and more preferably at least 97% thatof said feedstock. Unless otherwise specified, the term octane as usedherein refers to an (R+M)/2 octane, which is the sum of a material'sresearch octane and motor octane divided by 2.

[0100] The intermediate boiling second fractionation product of theeffluent from the olefin-modification reaction zone is contacted with aselective hydrotreating catalyst in the presence of hydrogen underconditions which are effective to selectively convert at least a portionof the sulfur in its sulfur-containing organic impurities to hydrogensulfide with minimum hydrogenation of olefins.

[0101] One such selective hydrotreating process called SCANfining™ islicensed by ExxonMobil Research and Engineering Company. The SCANfiningProcess is a catalytic desulfurization process that utilizes a catalystdesignated as RT 225 to selectively remove sulfur from fluidizedcatalytic cracking naphtha with minimum hydrogenation of olefins, herebypreserving octane. Yet another selective hydrotreating process is calledPRIME-G+™ and is licensed by IFP North America, Inc. This processenables over 98% desulfurization of FCC naphtha while maximizing octanebarrel by limiting olefin saturation. Another method for effecting theselective hydrotreating process in accordance with the process of thepresent invention is to contact the intermediate boiling second fractionwith conventional hydrotreating catalyst at selective hydrotreating zoneconditions which are generally less severe or milder than conventionalhydrotreating conditions. The selective hydrotreating zone conditionsinclude a temperature in the range of form about 100° C. to about 300°C., a pressure range form about 300 psig to about 600 psig, and a liquidhourly space velocity in the range of about 3 to about 10. The ratio ofhydrogen to hydrocarbon feed in the selective hydrotreating zone willrange from about 700 to about 2000 standard cubic feet per barrel offeed.

[0102] After the carrying out the selective hydrotreating step andremoving hydrogen sulfide, the intermediate boiling fraction will have asulfur content which is desirably less than about 50 ppm by weight,preferably less than 30 ppm by weight, and more preferably less than 10by weight. The octane of this intermediate fraction selectivehydrotreating process will desirably be at least 95 percent of that ofthe feedstock to the olefin-modification zone, preferably at least 97percent and most preferably 98 percent of that of the subject feedstock.

[0103] One embodiment of the invention is schematically illustrated inthe drawing. With reference to the drawing, total catalytic naphtha froma fluidized catalytic cracking process is passed through line 1 intopretreatment vessel 2. The naphtha feedstock is comprised of mixturehydrocarbons which include olefins, paraffins, naphthenes, andaromatics, and the olefin content is in the range from about 10 wt. % toabout 60 wt. %. In addition, the naphtha feedstock contains from about0.2 wt. % to about 0.5 wt. % sulfur in the form of sulfur-containingorganic impurities, which include thiophene, thiophene derivatives,benzothiophene and benzothiophene derivatives, mercaptans, sulfides anddisulfides. The feedstock also contains from about 5 to about 200 ppm byweight of basic nitrogen containing impurities.

[0104] The basic nitrogen containing impurities are removed from thefeedstock in pretreatment vessel 2 through contact with an acidicmaterial, such as an aqueous solution of sulfuric acid, under mildcontacting conditions which do not cause any significant chemicalmodification of the hydrocarbon components of the feedstock.

[0105] Effluent from pretreatment vessel 2 is passed through line 3 andis introduced into olefin-modification reactor 4, which contains anolefin-modification catalyst. The feed to reactor 4 passes through thereactor where it contacts the olefin-modification catalyst underreaction conditions which are effective to produce a product having abromine number which is lower than that of the feed from line 3. Inaddition, a substantial portion of the thiophenic and benzothiophenicimpurities are converted to higher boiling sulfur-containing materialincluding refractive sulfur compounds through alkylation by the olefinsin the feed.

[0106] The products from olefin-modification reactor 4 are dischargedthrough line 5 and are passed to distillation column 6 where theseproducts are fractionally distilled. A high boiling fraction, whichcomprises a hydrocarbon mixture which contains alkylatedsulfur-containing impurities including refractive sulfur compounds andleached catalyst compounds or components, is withdrawn from distillationcolumn 6 through line 7. An intermediate boiling fraction, which is ofreduced sulfur content relative to the sulfur content of the originalheavy naphtha feedstock and has a distillation endpoint less than about240 C., is withdrawn from distillation column 6 through line 8. Thelowest boiling fraction is withdrawn from distillation column 6 throughline 9.

[0107] The highest boiling third fraction from distillation column 6 ispassed through line 7 and is introduced into hydrodesulfurizationreactor 11, and hydrogen is introduced into reactor 11 through line 10.This third fraction is contacted with a hydrodesulfurization catalystwithin reactor 11 in the presence of hydrogen under conditions which areeffective to convert at least a portion of the sulfur in thesulfur-containing impurities of the feed from line 7 to hydrogensulfide. A product is withdrawn from reactor 11 through line 12 which,after removal of hydrogen sulfide, has a reduced sulfur content relativeto that of the feed from line 7. The sulfur content of this productwill, typically, be less than about 30 ppm by weight.

[0108] The intermediate boiling fraction from distillation column 6 ispassed through line 8 and is introduced into selective hydrotreatingreactor 14, and hydrogen is introduced into reactor 14 through line 13.The intermediate fraction is contacted with a selective hydrotreatingcatalyst within reactor 14 in the presence of hydrogen under conditionswhich are effective to convert at least a portion of the sulfur in thesulfur-containing impurities of the feed from line 8 to hydrogensulfide. A product is withdrawn from reactor 14 through line 15 which,after removal of hydrogen sulfide, has a reduced sulfur content relativeto both the heavy naphtha feedstock to the process and the feed fromline 8. The sulfur content of this product will, typically, be less thanabout 30 ppm by weight.

[0109] The lowest boiling first fraction is withdrawn from thedistillation column 6 through line 9. The sulfur content of thisfraction will typically be 10 ppm by weight.

[0110] The following examples are intended only to illustrate theinvention and is not to be construed as imposing limitations on theinvention.

EXAMPLE 1

[0111] A naphtha feedstock having the following analysis was contactedin an olefin-modification zone in accordance with the present invention.TABLE 11 S, ppm 580 Basic N, ppm less than 5 Total N, ppm 10 MercaptanS, ppm 53 RVP, psia 7.41 RON 92.4 MON 79.8 R + M/2 86.1 ASTM D86Distillation IBP ° C. 102.7 FBP ° C. 269.9 Peak Group Information, ppmThiophene 117.55 C1 Thiophene 253.58 C2 + Thiophenes 128.6

[0112] The olefin-modification zone consisted of two stages of fixed bedof solid phosphoric acid (obtained from Sud Chemie and sold under thename C84-5-01) and was operated at a temperature of 172° C. in the firststage and 122° C. in the second stage, a pressure of 500 psig and aliquid hourly space velocity of 1.5 LHSV.

[0113] The resulting olefin-modification reaction zone product wasfractionally distilled into three fractions in accordance with theprocess of the invention and two fractions for comparative purposes.

[0114] Distillations of the olefin-modification zone products werecarried out on a Fischer 800 Bench-scale semi-automatic distillationunit in accordance with the ASTM D2892 method.

[0115] The sample was heated in a three-liter flask with magneticstirrer, under a Nitrogen bleed.

[0116] Fractionation took place in a column of 18 mm diameter packedwith 4 mm Pro-pak gauze packing to give an efficiency of 15 theoreticalplates.

[0117] The vapor was liquefied on a condenser chilled to −20 C, and thedistillate taken-off at a ratio of 20:4 via a timed reflux divider intoa chilled receiver.

[0118] The temperature cut point was determined by measuring the vaportemperature with a resistance thermometer and associated electronicmeter.

[0119] The unit had the capacity to distil under vacuum, but in thiscase samples were distilled at atmospheric pressure, with temperaturescorrected to 760 mm.

[0120] Distillation products were purged with nitrogen and stored underrefrigeration prior to further testing.

[0121] The following Table III shows the relative amountsolefin-modification catalyst (“phosphorous”), total sulfur incomparative boiling range fractions: 100° C.−, and 100° C.+ and thethree boiling range fractions in accordance with the present invention:100° C.−, 100° C. to 200° C., and 200° C.+. TABLE III Two Cut Splittervs. Three Cut Splitter Product Contaminants Total Normal RefractivePhos- Sulfur Sulfur Sulfur phorous Nitrogen Bromine Yields (ppm) (ppm)(ppm) (ppm) (ppm) No. Wt % Feed 580 580 0 <0.2 10 84.9 100 Two CutSplitter OVHD: (IBP/100° C.) 10.6 10.6 0 <0.2 <0.3 62.3 49.9 Bottoms:(100° C.+) 1140 440 700 13.6 7.1 83.8 50.1 Three Cut Splitter OVHD(IBP/100° C.) 158 155 3 <0.2 <0.3 62.3 49.9 Sidedraw: (100° C./200° C.)359 352 7 <0.2 0.5 64.8 39.1 Bottoms: (200° C.) 3970 651 3319 61.1 33117 11

[0122] As can be observed form the above table the phosphorus andrefractive sulfur is preferentially concentrated in the highest boilingrange fraction when the olefin modification zone effluent is split intothree fractions in accordance with the present invention. This highestboiling range is of relatively small volume, i.e., 11% wt. % yield andtherefore permits a much smaller fraction of the olefin modificationzone product to being treated by the octane reducing,hydrodesulfurization process. Note in the comparative bottoms fraction,the yield is 50.1 wt. % which means that one half of theolefin-modification stream effluent would have to be hydrotreatednonselectively to remove refractive sulfur resulting in undesirableoctane loss for one-half of the stream verses 11% of the stream inaccordance with the process of the invention.

EXAMPLE 2

[0123] A naphtha feedstock having the following analysis was contactedin an olefin-modification zone in accordance with the present invention.TABLE IV S, ppm 450 Basic N, ppm less than 5 Total N, ppm 13 MercaptanS, ppm 4 RVP 11.12 RON 94.4 MON 80.1 R + M/2 87.3 ASTM D86 DistillationIBP ° C. 87.6 FBP ° C. 255.8 Peak Group Information, ppm Thiophene156.43 C1 Thiophenes 179.91 C2 + Thiophenes 37.4

[0124] The above feedstock was contacted in an olefin modification zonecomprising fixed bed of solid phosphoric acid (obtained from Sud Chemieand sold under the name C84-5-01). The olefin-modification zone wasoperated at a temperature of 193° C., a pressure of 250 psig and aliquid hourly space velocity of 1.5.

[0125] The reaction zone product was fractionated into three fractionsin accordance with the present invention and two fractions forcomparative purposes in a unit having the following characteristics: potvolume: ˜1500 gallons column height: 34′6″ column diameter: 12″ withstructured packing number of theoretical plates: 33

[0126] With respect to the three fractions, the product wasatmospherically distilled to a head temperature of 100° C. The take-offratios were 33 percent until head temperature equaled 80° C., 14 percentuntil head temperature equaled 90° C., 8 percent until head temperatureequaled 100° C. The system was then set for total reflux return. Thereceiver containing the IBP-100° C. fraction was cut. The system wasthen pulled under vacuum to 55 mmHG and take-off initiated. The take-offratios were between 5 percent and 8 percent during the 100-200° C. cut.The system was shutdown when the head temperature reached 106° C. at 55mmHG. The 100-200° C. fraction, pot fraction 200-FBP and vacuum trapfraction were cut.

[0127] The comparative two fraction splitting was carried out in ananalogous way resulting in a IBP to 100° C. fraction and a 100° C.+fraction.

[0128] The following table shows the sulfur distribution in therespective three fractions prepared in accordance with the presentinvention. It should be noted that the fractionation that was carriedout was less than ideal, because the IBP-100° C. fraction shows thepresence of C3+ thiophenes which would ordinarily not be present in thisfraction. TABLE V Sulfur Distribution Per Fraction Components IBP-100°C. 100° C.-200° C. 200° C.+ H2S 0 0 0 Mercaptans + 1 Coeluting 17.81 0 0Unknown Sulfides 3.17 6.33 0 Disulfides 0.72 0 0 Sulfoxides/Sulfones 0 00 Thiophene 11.16 0 0 Tetrahydrothiophene & Me- 1.45 20.08 0 ThiopheneC1 Thiophene 6.59 31.38 0 C2 Thiophene 2.58 66.14 0 C3 + C4 Thiophenes2.33 75.02 0.63 C5 Thiophene 3.72 179.93 12.39 C6 Thiophene 5.24 35.74919.06 C7 Thiophene 3.77 2.18 412.21 C8 + C9 Thiophene 2 0.25 410.2 C10Thiophene 2.19 0 549.25 C11 Thiophene 1.34 0 477.88 C12 Thiophene 0.74 0743.41 Unknowns 1.62 3.44 0

[0129] The comparative 100° C.+ fraction was subjected to conventionalhydrotreating at the following conditions:

[0130] 318° C.

[0131] 450 psig

[0132] 3 LHSV

[0133] The side-cut or 100° C. to 200° C. fraction obtained inaccordance with the present invention was subjected to a hydrotreatingstep at selective hydrotreating conditions including:

[0134] 307° C.

[0135] 450 psig

[0136] 3 LHSV

[0137] The hydrotreating unit was a fixed bed, downflow hydrotreatingpilot plant configured for once-through processing of naphtha ordistillate feeds at hydrogen pressures up to 2000 psig, hydrogen flowsto 5 scfh, liquid feed rates to 600 cc/hr and temperatures to 800 deg.F. The reactor was approximately 0.96″ id×18″ long and can hold up to a120 cc of catalyst charge and is heated by a salt bath. The internalcatalyst bed temperature was monitored by a programmable traversingsingle point thermocouple. An LDC ConstaMetric 3200 precision meteringpump pumps the feed in and hydrogen flow into the unit through a Brooksmass flow meter. The combined liquid and gas flows were passed throughthe downflow reactor and are separated in a Strahman sight gage glass.The offgas pressure is controlled via a Rosemount pressure transmitterand Badger control valve where the offgas flow goes through a causticscrubber and is measured downstream at atmospheric pressure via anAlexander Wright wet test meter. The liquid product in the separator hada level control via Rosemount differential transmitter and a Badgercontrol valve. The product was cooled via a tube in tube heat exchangerand collected in a refrigerator through an automated valve manifold,sequenced by computer control into one of three product receivers. Thecomputer control/data collection was done via Analog Devices uMac 6000and automated safeguard shutdowns were controlled via a Siemens SimaticTI505 PLC.

[0138] Table VI below shows the octane retention afforded by theinvention while effecting deep desulfurization. Note that the sulfurlevel in the combined overhead “OVHD” and intermediate fraction“Sidedraw” would have been significantly lower had the fractionationbeen carried out ideally. Specifically, the combined first and secondfractions (“OVHD” and “Sidedraw”) show an octane loss of 87.3 to 84.5with a desulfurization of down to 50 ppm, wt, while the comparativeprocess only desulfurizes to 60 ppmw for a similar octane loss. Theresults would have been more significant had the fractionation been moreideal. Further, in order to achieve the product sulfur in thecomparative combined fraction, the hydrodesulfurization conditions wouldhave to be substantially more severe and would be:

[0139] 332° C.

[0140] 450 psig

[0141] 1.5 LHSV

[0142] These more severe conditions would result in further octane losson 33.1% of the stream. TABLE VI Two Cut Splitter vs. Three Cut SplitterOctane Retention Sulfur Wt. % (ppm) Octane Yields Feed 450 87.3 100 TwoCut Splitter OVHD: (IBP/100° C.) 69 86.1 66.9 Bottoms: (100° C.+) 7082.4 33.1 Combined Product 69 84.9 100 Three Cut Splitter OVHD:(IBP/100° C.) 69 86.1 66.9 Sidedraw: (100° C./200° C.) 15 80 24.6Bottoms: (200° C.) 8.5 OVHD/SD Combined Product 50 84.5 91.5

We claim:
 1. A process for producing a product of reduced sulfur contentfrom a feedstock, wherein said feedstock contains sulfur-containingorganic impurities and is comprised of a normally liquid mixture ofhydrocarbons which includes olefins, said process comprising: (a)contacting the feedstock with an olefin-modification catalyst in atleast one olefin-modification reaction zone under conditions which areeffective to produce a product having a bromine number which is lowerthan that of the feedstock and wherein the product contains refractivesulfur compounds; (b) fractionating the product from saidolefin-modification reaction zone to produce: (i) a first fraction whichcomprises sulfur-containing organic impurities and has a distillationendpoint which less than about 140° C.; (ii) a second fraction which ishigher boiling than the first fraction which comprises sulfur-containingorganic impurities and has a distillation endpoint which is less thanabout 240° C.; and (iii) a third fraction which is higher boiling thanthe second fraction and comprises sulfur-containing organic impuritiesand refractive sulfur compounds; (c) contacting said second fractionwith a selective hydrotreating catalyst in the presence of hydrogen in aselective hydrotreating reaction zone under conditions which areeffective to convert at least a portion of the sulfur as containingimpurities in the second fraction to hydrogen sulfide; and (d)contacting said third fraction with a hydrodesulfurization catalyst inthe presence of hydrogen in a hydrodesulfurization reaction zone underconditions which are effective to convert at least a portion of thesulfur in said sulfur-containing impurities of the third fraction tohydrogen sulfide.
 2. The process of claim 1 wherein the feedstockcontains paraffins and wherein the conditions in saidolefin-modification reaction zone are effective to produce a producthaving a bromine number which is lower than that of the feedstock andless than 10% of the paraffins in the feedstock are cracked.
 3. Theprocess of claim 1 which additionally comprises removing hydrogensulfide from the effluent of said selective hydrotreating reaction zoneto yield a desulfurized product which contains less than about 50 partsper million by weight of sulfur.
 4. The process of claim 3 wherein theoctane of the desulfurized product is at least 95% that of the feedstockto the olefin-modification reaction zone.
 5. The process of claim 1wherein the feedstock contains from about 0.05 wt. % to about 0.7 wt. %of sulfur in the form of organic sulfur compounds.
 6. The process ofclaim 1 which additionally comprises removing hydrogen sulfide from theeffluent of said hydrodesulfurization reaction zone to yield adesulfurized product which contains less than about 50 parts per millionby weight sulfur.
 7. The process of claim 6 wherein the octane of thedesulfurized product is at least 90% that of the feedstock to theolefin-modification reaction zone.
 8. The process of claim 1 whereinsaid feedstock contains basic nitrogen-containing impurities and saidprocess additionally comprises removing said basic nitrogen-containingimpurities from the feedstock before it is contacted with theolefin-modification catalyst.
 9. The process of claim 8 wherein saidfeedstock is comprised of hydrocarbons from a catalytic crackingprocess.
 10. The process of claim 1 wherein said feedstock issubstantially free of basic nitrogen-containing impurities.
 11. Theprocess of claim 1 wherein said feedstock is comprised of a mixture ofhydrocarbons which boils in the gasoline range.
 12. The process of claim1 wherein the feedstock is comprised of a treated naphtha which isprepared by removing basic nitrogen-containing impurities from a naphthaproduced by a catalytic cracking process.
 13. The process of claim 1wherein the third fraction ranges from 2 to 10 vol % of the product fromthe olefin-modification reaction zone.
 14. The process of claim 1wherein the feedstock has an initial boiling point which is below about79° C. and distillation endpoint which is not greater that about 345° C.15. A process for producing products of reduced sulfur content from afeedstock, wherein said feedstock contains sulfur-containing organicimpurities and is comprised of a normally liquid mixture of hydrocarbonswhich includes olefins, said process comprising: (a) contacting thefeedstock with an olefin-modification catalyst in an olefin-modificationreaction zone under conditions which are effective to produce a productwhich has a lower bromine number than that of the feedstock wherein theproduct contains refractive sulfur compounds, wherein saidolefin-modification catalyst is selected from the group consisting ofall solid acid catalysts; (b) fractionating the product from saidolefin-modification reaction zone to produce: (i) a first fraction whichcomprises sulfur-containing organic impurities and has a distillationendpoint which is less than about 120° C.; (ii) a second fraction whichis higher boiling than the first fraction which comprisessulfur-containing organic impurities and has a distillation endpointwhich is less than about 200° C.; and (iii) a third fraction which ishigher boiling than the second fraction and comprises sulfur-containingorganic impurities and refractive sulfur compounds; (c) contacting saidsecond fraction with a selective hydrotreating catalyst in the presenceof hydrogen in a selective hydrotreating reaction zone under conditionswhich are effective to convert at least a portion of the sulfur in saidsulfur containing impurities in the second fraction to hydrogen sulfide;(d) contacting said third fraction with a hydrodesulfurization catalystin the presence of hydrogen in a hydrodesulfurization reaction zoneunder conditions which are effective to convert at least a portion ofthe sulfur in said sulfur-containing impurities of the third fraction tohydrogen sulfide.
 16. The process of claim 15 which additionallycomprises removing hydrogen sulfide from the effluent of said selectivehydrotreating reaction zone to yield a desulfurized product having anoctane which is at least 98% that of the feedstock to theolefin-modification reaction zone.
 17. The process of claim 15 whichadditionally comprises removing hydrogen sulfide from the effluent ofsaid hydrodesulfurization reaction zone to yield a desulfurized producthaving an octane which is at least 97% that of the feedstock to theolefin-modification reaction zone.
 18. The process of claim 15 whereinsaid feedstock is comprised of hydrocarbons from a catalytic crackingprocess.
 19. The process of claim 15 wherein the feedstock is comprisedof a treated naphtha which is prepared by removing basicnitrogen-containing impurities from a naphtha produced by a catalyticcracking process.